Photoinduced charge separation of phenothiazine derivatives in layered zirconium phosphate at room temperature

Abstract

The photoionization of alkylphenothiazines in layered α-zirconium phosphate (α-ZrP or ZrP) has been studied by electron spin resonance (ESR) and diffuse reflectance spectroscopies. Alkylphenothiazines (PC_n where n=1, 2, 3, 4, 6, 8, 10, 16) were synthesized and used to study the effects of the alkyl chain length. Phenothiazine and N-alkylphenothiazines were incorporated into ZrP by impregnation and ion-exchange methods. By both methods N-alkylphenothiazines are only incorporated onto the external surface of zirconium phosphate rather than into the interlayer space. The alkylphenothiazines photooxidize at room temperature to form stable alkylphenothiazine cation radicals (PC_n⁺) which are measured by ESR and diffuse reflectance. The framework of ZrP is suggested to be the electron acceptor. Both the photoyield and the decay rate of the alkylphenothiazine cation radical depend on the alkyl chain length. As the alkyl chain length of the PC_n⁺ cation radical increases from methyl to hexyl, the photooxidation yield increases; and as the alkyl chain length increases further from hexyl to hexadecyl, the photooxidation yield decreases. However, the decay rate of the PC_n⁺ cation radical gradually increases from methyl to hexadecyl which is explained in terms of a greater inductive effect for longer alkyl chains on the PC_n⁺ cation radicals. Also, the resolution of the ESR spectrum decreases with increasing alkyl chain length which is interpreted as being due to decreasing mobility of the radical which should also decrease the decay rate. Both impregnation and ion-exchange methods are reasonably effective for incorporating alkylphenothiazine molecules into ZrP for efficient photooxidation. The photoyields of N-alkylphenothiazines in ZrP are larger than in silica gel which suggests that ZrP assemblies can be utilized for solar energy conversion and storage.

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